The present invention relates generally to iron-containing glass articles and glass compositions and, more particularly, to a method of increasing the redox ratio of iron in glass articles and glass compositions.
In conventional float glass production operation, a batch of glass forming components is heated in a furnace to melt the glass forming components into a liquid mass. Furnaces for glass batch melting include conventional furnaces that employ chemical refining and overhead heating or other special glass melting furnaces, such as vacuum or physical refining furnaces. The melted batch components react to produce a glass which is then removed from the furnace and deposited onto a pool of molten tin. The glass is formed, processed and cooled to form solid glass articles. A basic glass batch mixture includes sand, soda ash, dolomite, limestone and sulfate (e.g. salt cake or gypsum). Additional materials may also be added which affect the final properties, e.g., color, transmittance, reflectance, optical properties etc., of the glass and/or melting characteristics of the batch. Known glass batch compositions are disclosed in U.S. Pat. Nos. 5,688,727; 5,352,640; 5,545,596; 5,837,629; 5,807,417; and 5,780,372, which are herein incorporated by reference. For example, coloring agents may be added to the batch components to achieve a desired final color of the glass. The particular batch components used and their relative amounts are selected based on the desired properties of the finished glass article and desired melting characteristics of the batch.
In order to manufacture glass having solar control properties, such as heat absorbing or infrared absorbing glass, solar control materials, such as iron-containing materials may be added to the batch forming components. The iron may be present in both the ferrous, e.g., FeO, and ferric, e.g., Fe2O3, states. However, the relative amounts of ferrous and ferric iron in the final glass article have a direct effect on the color and also the optical and solar control properties of the glass article. For example, ferrous iron absorbs light in the infrared and visible regions and generally produces glass having a blue color. Ferric iron absorbs light in the ultraviolet and visible regions and generally produces glass having a yellow color. Thus, the solar control properties of the glass, e.g., transmittance, reflectance, absorbance, etc., can depend upon the amounts of ferrous and ferric iron in the glass.
Rouge is well known and is conventionally used as a convenient source of iron when manufacturing solar control glass. However, rouge consists principally of ferric iron. Therefore, in order to produce infrared absorbing glass, it is necessary to chemically reduce at least a portion of this ferric to ferrous iron. This reduction is typically done by adding a carbon source, e.g., coal, graphite, sugar solution, etc., to the batch of glass forming components to facilitate the reduction of ferric iron to ferrous iron to increase the xe2x80x9credox ratioxe2x80x9d of the glass, as defined hereinbelow. For high performance, infrared absorbing glass articles and compositions, redox ratios greater than about 0.3 are desirable. However, it is very difficult to obtain such a high redox ratio utilizing currently available glass production methods due principally to heat transfer difficulties. For example, in overhead heated furnaces, a layer of silica scum may form on top of the melt when large amounts of coal are added. The scum layer inhibits thermal transfer and hence may make it difficult to adequately heat the interior of the melt.
To increase the redox ratio, typically the amount of oxidizing agent, e.g., sulfate material such as salt cake or gypsum, added to the batch is decreased and large amounts of both rouge and carbon are added to the glass batch mixture to achieve a desired higher level of ferrous iron in the batch and hence in the resulting glass article. However, a major drawback of decreasing sulfate addition and adding more carbon is that the melting quality of the batch becomes poorer and the formation of silica scum increases, which in turn further reduces thermal transfer into the glass melt and can lead to unmelted silica stone defects. This places a practical limit on the total amount of carbon that can be added. Additionally, since the carbon is typically added in the form of coal, an increase in the amount of coal may also increase the amount of gaseous sulfur byproducts, e.g., SO2, which may be produced during the glass making process due to a reaction between salt cake or gypsum and the coal. Treatment of such sulfur byproducts before release into the atmosphere may increase the cost of glass production. Further, while the addition of more rouge does lead to a higher amount of ferrous iron, it also produces a higher amount of ferric iron in the glass article and a higher total iron concentration in the glass. As the total iron concentration increases, the visible light transmittance of the glass decreases. For some commercial applications such as automotive windshield transparencies or architectural windows, it is desirable to produce a glass product which is infrared absorbing but also has a relatively high visible light transmittance, i.e., a glass product having a low total iron content which is highly reduced from Fe2O3 to FeO and with a transmittance of greater than about 60%, preferably greater than about 70% for windshields or sidelights for use in the United States.
Attempts have been made to produce glass products having good infrared absorption, i.e., glass which is high in ferrous iron, but which also has good visible light transmittance. For example, U.S. Pat. No. 5,478,783, herein incorporated by reference, discloses a glass production method in which selenium and cobalt containing coloring agents are added to the batch and wuestite is used instead of rouge as the source of Fe2O3. However, the degree of reduction is critical and must not be greater than 21.34%. Higher reduction causes the glass to become too dark and the melting process difficulty increases due to poor heat penetration into the melt.
U.S. Pat. Nos. 5,523,263 and 5,641,716, herein incorporated by reference, disclose glass production methods in which ilmenite (FeTiO3) is added to the batch as a source of titanium and as a partial source of Fe2O3.
EP 765,846 and U.S. Pat. No. 5,888,264, herein incorporated by reference, disclose a method of preparing ferrous containing glass by adding a fayalite containing material to the batch. However, no disclosure is given regarding the relationship between fayalite and coal as glass batch components or how the fayalite affects the final redox ratio.
It would be advantageous to provide a method of making glass and a glass product having a relatively high redox, e.g., greater than about 0.3, ratio with a total iron content of about 0.2-2.0 wt. %, preferably 0.25-1 wt. %, where the method does not have the drawbacks of the above-discussed conventional glass making procedures. It would also be advantageous to provide a glass making method which reduces or eliminates the need for the addition of carbon, e.g., coal, to the glass forming components.
Glass is made from batch components having a source of ferrous iron added to the batch components to give a concomitant higher level of ferrous iron in the resulting glass product. Producing glass with a redox ratio above about 0.25 is an aspect of the invention. The iron source is preferably an iron silicate material, such as fayalite (2FeO.SiO2), iron garnet (3FeO.Fe2O3.3SiO2), magnesium-iron olivine (2(Mg,Fe)O.SiO2), grunerite (6FeO.8SiO2.Fe(OH)2), actinolite (CaO.3(Mg,Fe)O.4SiO2) or iron rich anthophyllite ((Mg,Fe)O.SiO2) or any combination thereof. The presence of a ferrous iron source in the glass batch components decreases or eliminates the need for carbon and also leads to a glass article having a relatively high redox ratio, e.g., greater than about 0.25. Due to the presence of silicate in the iron source, the amount of sand present in the batch is preferably reduced in proportion to the amount of silicate present in the added iron source. Glass compositions formed from such batch components may have a total iron amount in the range of about 0.2-2.0 wt. of the glass composition.
A suitable basic batch composition for forming a soda-lime glass article of the invention generally can include about 48 to 72 wt. % silica material, e.g., sand; about 14 to 28 wt. % of a sodium material such as soda ash; about 0 to 19 wt. % of a calcium and magnesium material such as dolomite; about 0 to 15 wt. % of a calcium carbonate material such as limestone, and about 0.15 to 6.50 wt. % of a ferrous iron source, e.g., a ferrous silicate source such as fayalite. It should be appreciated that the batch composition may include other typical soda-lime glass batch additives or small amounts of other materials, such as an oxidizing material, such as a sulfate material, e.g., about 0-2.3 wt. % of gypsum, melting and refining aids, tramp materials or impurities. It should be further appreciated that additional materials may be added to color the glass and/or improve its solar performance. Rouge may also be added to the batch composition to increase the ferric iron and total iron content as desired. The batch may also have one or more colorant components such as elements or compounds of Ti, Se, Co, Cr, Ni, Mn, Ce, V, Mo or Cu intentionally added to modify the color of the glass, particularly with higher redox. Also, the batch may have 0 to 95 wt. of cullet of any type known to those skilled in the art. The batch is preferably substantially free of ferric iron. The batch may further include Al2O3 containing materials such as nepheline syenite or feldspar to produce glass having about 1xc2xd wt. % alumina to improve the chemical and stain durability of the glass.